Aims A century of atmospheric deposition of sulfur and nitrogen has acidified soils and undermined the health and recruitment of foundational tree species in the northeastern US. However, effects of acidic deposition on the forest understory plant communities of this region are poorly documented. We investigated how forest understory plant species composition and richness varied across gradients of acidic deposition and soil acidity in the Adirondack Mountains of New York State. Methods We surveyed understory vegetation and soils in hardwood forests on 20 small watersheds and built models of community composition and richness as functions of soil chemistry, nitrogen and sulfur deposition, and other environmental variables. Results Community composition varied significantly with gradients of acidic deposition, soil acidity, and base cation availability (63% variance explained). Several species increased with soil acidity while others decreased. Understory plant richness decreased significantly with increasing soil acidity (r = 0.60). The best multivariate regression model to predict richness (p < 0.001, adjusted-R 2 = 0.60) reflected positive effects of pH and carbonto-nitrogen ratio (C:N). Conclusions The relationship we found between understory plant communities and a soil-chemical gradient, suggests that soil acidification can reduce diversity and alter the composition of these communities in northern hardwood forests exposed to acidic deposition.
AimsClimate change is expected to shift climatic envelopes of temperate tree species into boreal forests where unsuitable soils may limit range expansion. We studied several edaphic thresholds (mycorrhizae, soil chemistry) that can limit seedling establishment of two major temperate tree species, sugar maple and American beech.
MethodsWe integrate two eld surveys of tree seedling density, mycorrhizal colonization, and soil chemistry in temperate deciduous and montane conifer forests of the Adirondack and Green Mountains in northeastern United States. We conducted correlation and linear breakpoint analyses to detect soil abiotic and biotic thresholds in species seedling distributions across edaphic gradients.
ResultsIn the Green Mtns, sugar maple seedling relative importance (IV) declined sharply at low pH (<3.74 in upper mineral soil) and low mycorrhizal colonization (<27.5% of root length colonized). Sugar maple IV was highly correlated with soil chemistry, while beech was generally insensitive to soil variables. Mycorrhizal colonization of sugar maple was strongly positively correlated with soil pH and conspeci c overstory basal area. In the Adirondacks, sugar maple IV plateaued above thresholds in soil calcium (~2 meq/100g) and magnesium (~0.3 meq/100g) where these nutrients were no longer limiting. Sugar maple IV declined steeply with increasing aluminum and decreasing pH.
ConclusionsThe establishment of sugar maple, but not beech, was impeded by both biotic and abiotic soil components in boreal conifer forests and by soil acidity in temperate deciduous forests. These differences in species sensitivity to edaphic thresholds will likely affect species success and future shifts in forest composition.
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